1. Field of the Invention
The present invention relates to an OFDM reception apparatus for receiving a transmission signal based on an OFDM modulation scheme and, more particularly, to FFT window control and clock reproduction control at the time of receiving an OFDM signal.
2. Description of Related Art
Development of a digital modulation scheme has recently been widespread in transmission of audio signals and video signals. In digital terrestrial broadcasting, in particular, an orthogonal frequency division multiplexing (hereinafter abbreviated as OFDM) modulation scheme with features such as robustness against multipath interference and high spectral efficiency has attracted attention.
In digital terrestrial broadcasting in Japan, scattered pilot signals (hereinafter referred to as SP signals) are sparsely inserted in a frequency direction and in a time direction. A reception apparatus corrects a transmission path distortion based on such SP signals and performs data demodulation. An SP signal is a signal whose amplitude and phase are constant and is used to estimate variations in an amplitude and a phase of each type of carrier and generate a correction signal. A TMCC (Transmission and Multiplexing Configuration Control) signal for providing a hierarchical configuration of a signal to be transmitted and transmission parameters to a reception apparatus and an AC (Auxiliary Channel) signal for transmitting additional information on transmission control are arranged randomly in the frequency direction, are subjected to differential binary phase shift modulation (hereinafter referred to as differential modulation or DBPSK modulation), and are transmitted. The ground-wave digital signals are described in detail in “ARIB STD-B31” by the Association of Radio Industries and Businesses.
In a conventional OFDM reception apparatus, an SP signal is also used in FFT window control or clock reproduction control. After SP signals are extracted from an FFT output, the SP signals are converted into signals at intervals of three carriers by interpolation in a time axis direction. The signals are then subjected to inverse Fourier transform processing to produce time-direction signals. A peak position of an inverse Fourier transform output is detected (the detection is referred to as delay profile detection), and FFT window control or clock reproduction control is performed such that the peak position is located at a predetermined position.
However, even if SP signals are subjected to interpolation in the time axis direction, signals at intervals of three carriers in a frequency direction are produced. Accordingly, in a transmission path where there may be a delayed wave with a delay longer than ⅓ of an effective symbol length of an OFDM signal, a delayed wave occurs as an aliasing component in an inverse Fourier transform output. If control is performed using a peak based on such an aliasing component in a situation where the aliasing component occurs, it is impossible to normally perform FFT window control or clock reproduction control.
A technique for allowing correct FFT window control even in an environment including an interference wave outside a range detectable by an SP carrier is disclosed as a conventional technique for an OFDM reception apparatus in Japanese Patent Application Laid-Open Publication No. 2007-324905.
However, the technique in Japanese Patent Application Laid-Open Publication No. 2007-324905 does not solve the problem of an inability to normally perform FFT window control or clock reproduction control due to an aliasing component generated in an inverse Fourier transform output if there is a delayed wave with a delay longer than ⅓ of the effective symbol length of an OFDM signal.